Near-Infrared Time-Resolved Spectroscopy for Assessing Brown Adipose Tissue Density in Humans: A Review

Brown adipose tissue (BAT) mediates adaptive thermogenesis upon food intake and cold exposure, thus potentially contributing to the prevention of lifestyle-related diseases. 18 F-fluorodeoxyglucose (FDG)–positron emission tomography (PET) with computed tomography (CT) ( 18 FDG–PET/CT) is a standard method for assessing BAT activity and volume in humans. 18 FDG–PET/CT has several limitations, including high device cost and ionizing radiation and acute cold exposure necessary to maximally stimulate BAT activity. In contrast, near-infrared spectroscopy (NIRS) has been used for measuring changes in O 2 -dependent light absorption in the tissue in a non-invasive manner, without using radiation. Among NIRS, time-resolved NIRS (NIR TRS ) can quantify the concentrations of oxygenated and deoxygenated hemoglobin ([oxy-Hb] and [deoxy-Hb], respectively) by emitting ultrashort (100 ps) light pulses and counts photons, which are scattered and absorbed in the tissue. The basis for assessing BAT density (BAT-d) using NIR TRS is that the vascular density in the supraclavicular region, as estimated using Hb concentration, is higher in BAT than in white adipose tissue. In contrast, relatively low-cost continuous wavelength NIRS (NIR CWS ) is employed for measuring relative changes in oxygenation in tissues. In this review, we provide evidence for the validity of NIR TRS and NIR CWS in estimating human BAT characteristics. The indicators (Ind NIRS ) examined were [oxy-Hb] sup , [deoxy-Hb] sup , total hemoglobin [total-Hb] sup , Hb O 2 saturation (StO 2sup ), and reduced scattering coefficient ( μ s sup ′ ) in the supraclavicular region, as determined by NIR TRS , and relative changes in corresponding parameters, as determined by NIR CWS . The evidence comprises the relationships between the Ind NIRS investigated and those determined by 18 FDG–PET/CT; the correlation between the Ind NIRS and cold-induced thermogenesis; the relationship of the Ind NIRS to parameters measured by 18 FDG–PET/CT, which responded to seasonal temperature fluctuations; the relationship of the Ind NIRS and plasma lipid metabolites; the analogy of the Ind NIRS to chronological and anthropometric data; and changes in the Ind NIRS following thermogenic food supplementation. The [total-Hb] sup and [oxy-Hb] sup determined by NIR TRS , but not parameters determined by NIR CWS , exhibited significant correlations with cold-induced thermogenesis parameters and plasma androgens in men in winter or analogies to